1. Field of the Invention
This invention relates to an air flow meter for metering the flow rate of intake air of an internal combustion engine and, more particularly, to an air flow meter having a flow metering heat-sensitive resistor located in an auxiliary passage which is in turn arranged in a main passage forming an intake air passage.
2. Description of the related art
A known hot-wire air flow meter for an internal combustion engine, as disclosed in U.S. Pat. No. 4,887,577 has a body forming a main passage defining an intake air passage of an internal combustion engine; an auxiliary passage forming member arranged in the main passage and forming an auxiliary passage having a longitudinal passage in the axial direction of the main passage leading to transverse passages formed in the radial direction of the main passage; and a heat-sensitive resistor arranged in the longitudinal passage of the auxiliary passage for metering the flow rate of intake air. The resistor is located in a radial holder across the main passage which must necessarily be transversely thickened to accommodate the resistor across the air flow passage. So as to provide a symmetrical construction across the main passage the auxiliary passage forming member is thickened transversely across the longitudinal passage at the opposite side of the holder of the heat-sensitive resistor, as shown in FIG. 4 of the said U.S. patent, so as to provide support for the portion defining tile auxiliary passage.
JP-A-61-53518 (1986) and JP-A-64-10127 (1989) disclose the holder of the heat-sensitive resistor constructed such that a protecting member for the heat-sensitive resistor is disposed around the heat-sensitive resistor.
The member forming the auxiliary passage necessarily has its internal shape and size determined according to the air flow specification requirement of the auxiliary passage and since the outer wall forms part of the main passage, its external shape and size exert serious influences upon the air flow of the main passage. If this external shape is not correct, an increase in pressure loss occurs, and vortexes are established in the air flow. As a result, the hot-wire air flow meter output fluctuates (herein termed "output noise") which results in a decrease in accuracy of a fuel injection unit controlled thereby.
The prior art thus far described has failed to take into consideration the need for a uniform thickness and reduction in thickness of the auxiliary passage forming member but has determined tile internal and external shapes and sizes from the above-specified necessary specifications of the auxiliary passage and the main passage. As a result, the auxiliary passage forming member has tended to have a thick portion extending transversely to the main passage which causes a problem in that the product weight is increased. Even more importantly, because the body and the auxiliary passage forming member are integrally molded, if the auxiliary passage forming member is solid and relatively thick both axially and transversely of the main passage, and if it is made of a synthetic resin, a large shinkage normally occurs due to molding shrinkage, which then causes another problem in that the desired shape and size cannot be obtained resulting in decreased accuracy (see FIG. 5 described later herein). If the auxiliary passage forming member is made of a metal, such as aluminum, a similar but less serious problem arises than when synthetic resin is used.
If the thickness of the auxiliary passage forming member is to be made uniform, on the other hand (as shown in FIG. 6 described later herein), the external shape is incorrect to raise a further problem in that the aforementioned pressure loss and output noise occur due to the member having a flat leading surface. Also, such a construction is not symmetrical across the main passage since the axial length of the resistor holder is greater than that of the auxiliary passage forming member.
Moreover, the prior art technologies, as disclosed in JP-A-61-53518 (1986) and JP-A-64-10127 (1989), are directed to the protecting member for the heat-sensitive resistor, and neither has the object of improving the shape and size accuracies of the auxiliary passage forming member.
The present invention seeks to overcome the foregoing disadvantages.
A first specific object of the present invention is to provide an air flow meter in which the auxiliary passage forming member has reduced wall thickness without increasing the pressure loss and the output noise.
A second specific object of the present invention is to provide an air flow meter which can not only eliminate the aforementioned thick portion but also protect the heat-sensitive resistor arranged in the auxiliary passage.
A third specific object of the present invention is to provide an air flow meter which can not only eliminate the aforementioned thick portion but also reduce the output error caused by the intake pulsations of an internal combustion engine.
A fourth specific object of the present invention is to provide an air flow meter which can not only eliminate the aforementioned thick portion but also reduce the fluctuations of the air flow in the auxiliary passage and the meter output noise.